Defining high aspect ratio structures with controllable sidewalls in silicon has become increasingly important both in the nanometer and micrometer scale for solar cells, microelectronic devices, and chemical analysis (see, for example, references 1-5). High aspect ratio micrometer pillars are used for solar cell investigations while nanometer scale high aspect ratio pillars are enabling fundamental investigations in theories of nanopillar stress mechanics (see, for example, reference 5), silicon based lasers, and nanoelectronic devices such as finFETs (see, for example, reference 2). Currently various nanofabrication techniques exist that rely on self assembly or bottom-up processing (see, for example, reference 6). Some top-down processing enabling reproducibility in nanofabrication can be found, for example, in references 7-8.
Further applications are high surface area chemical sensors (see, for example, reference 3) mechanical oscillators (see, for example, references 15-16) and piezo-resistive sensors. High aspect ratio pillars with diameters between 50-100 nm could prove useful for core-shell type plasmonic resonators (see, for example, reference 17) while pillars with sub-10 nm diameters have shown promising light emission characteristics (see, for example, references 18-19).
When fabricating high aspect ratio nanopillars and micropillars, smooth sidewalls with controllable angles, and precision patterning is highly desirable. Moreover, high reproducibility and cost reduction involves reliable reproduction of identical structures and critical positioning of such structures (see, for example, references 20-21, for experimental applications). Achieving these criteria has been greatly enhanced by inductively coupled plasma (ICP) etching techniques. Etches utilizing etch/passivation chemistries such as the cryogenic silicon etch (see, for example, reference 9) and SF6/C4F8 silicon etch (see, for example, reference 10), have enabled smooth and controllable sidewalls. Using electron beam lithography, resolution has improved to the tens of nanometers. Transferring such features to a substrate involves high quality of etch masks (see, for example, references 11-12). High selectivity of the resist will result in deeper etched features. A thicker resist may be used to achieve a greater etch depth trading off the maximum achievable resolution. A previously reported selectivity for cryogenic etch mask is 100:1 when using photoresist and 200:1 when using silicon dioxide (see, for example, reference 14).
Mechanical manipulation of micropillars and nanopillars using electro-static actuation (see, for example, references 20-21) involves a continuous supply of power to maintain deformation. A strain of less than 3% can be achieved (see, for example, reference 22-23) using deformation methods such as pseudomorphic growth. Given its impact on electronic as well as optical properties (see, for example, references 23-25), methods to accurately control strain are becoming increasingly popular in modern devices.
Anisotropic strain has recently been exploited as a method for breaking the inversion symmetry in silicon photonics (see, for example, references 24-25) introducing a second order non-linearity. Furthermore, such asymmetrically strained materials can exhibit interesting optical selection rules (see, for example, reference 23) based on the strained splitting of the degenerate light and heavy hole bands. Previously proposed methods to introduce strain typically rely on the deposition of lattice mismatched layers, a method which can incorporate a few percent of strain, and which is fixed at fabrication time (see, for example, references 22-23).